This research targets structure-function relationships in selected metalloenzymes, metalloproteins, and metal-containing biomaterials, using synchrotron radiation-based X-ray absorption spectroscopy (XAS). XAS provides direct local structural information about metal and metalloid (e.g., Se, As) coordination environments and also probes the electronic structure (e.g., oxidation states) of these sites in amorphous (non-crystalline) biological systems. Specific applications include: (1) the relationship of metal coordination environment to DNA binding ability of a number of metalloregulatory and metallosensor proteins, especially those in the ArsR/SmtB and MerR families. This application seeks to understand the molecular mechanisms of organism response to metals in the cellular environment, both toxic heavy metals and essential metals. Several examples of metal sensors and metal transporters involved in metal homeostasis will be characterized. (2) the molecular mechanisms of the essential micronutrient selenium in its biological interactions. First, we will study the mechanism of selenophosphate synthetase, involved in the first committed step of selenocysteine incorporation into proteins. Second, we will use Se substitution for S as a spectroscopic probe to understand the involvement of Fe-S clusters in S-adenosylmethionine (SAM)-dependent radical generation in this enzyme super-family. (3) the molecular environment of metal-halogen biomaterials that are deposited in arthropod (and other) cuticular structures (in jaws, claws, stings, etc.). These biomaterials are non-crystalline and very hard but their detailed structure and biosynthesis is unknown. XAS microprobe methods will be used to investigate the micrometer-scale structure of these materials. Parallel to these XAS applications, a next-generation approach to EXAFS data analysis will be pursued to integrate computational chemistry as a quantitative surrogate for "chemical intuition" in interpreting EXAFS in terms of molecular structure. Public health depends on understanding the nutritional and toxicological effects of metals in the human diet and environment. The human organism must be able to maintain optimal levels of a variety of metals (as trace elements) in their proper place within tissues and cells. Understanding how both essential and toxic metals interact with biological systems will lead to nutritional and pharmacological advances.